Driven by a desire to find new structural polysaccharides using enzymatic syntheses or genetic engineering of microorganisms or plant hosts, researchers have discovered polysaccharides that are biodegradable and can be made economically from renewably sourced feedstocks. One such polysaccharide is poly alpha-1,3-glucan, a glucan polymer characterized by having alpha-1,3-glycosidic linkages. This polymer has been isolated by contacting an aqueous solution of sucrose with a glucosyltransferase (GTF) enzyme isolated from Streptococcus salivarius (Simpson et al., Microbiology 141:1451-1460, 1995).
U.S. Pat. No. 7,000,000 disclosed the preparation of a polysaccharide fiber using an S. salivarius gtfJ enzyme. At least 50% of the hexose units within the polymer of this fiber were linked via alpha-1,3-glycosidic linkages. S. salivarius gtfJ enzyme utilizes sucrose as a substrate in a polymerization reaction producing poly alpha-1,3-glucan and fructose as end-products (Simpson et al., 1995). The disclosed polymer formed a liquid crystalline solution when it was dissolved above a critical concentration in a solvent or in a mixture comprising a solvent. Continuous, strong, cotton-like fibers were obtained from this solution that could be spun and used in textile applications.
Not all glucosyltransferase enzymes can produce glucan with a molecular weight and percentage of alpha-1,3 glycosidic linkages suitable for use in spinning fibers. For example, most glucosyltransferase enzymes do not produce glucan having at least 50% alpha-1,3 glycosidic linkages and a number average degree of polymerization of at least 100. Therefore, it is desirable to identify glucosyltransferase enzymes that can convert sucrose to glucan polymers having a high percentage of alpha-1,3 glycosidic linkages and high molecular weight.
Reactions are disclosed herein that comprise glucosyltransferase enzymes containing certain amino acid motifs. These enzymes can synthesize high molecular weight, linear alpha-1,3-glucan polymer. Also disclosed are methods for identifying such enzymes.